1) Support for nested folders
2) SD card
3) User-replaceable battery
4) Support for a wide range of non-proprietary, non-DRMed formats, including fb2

The first three are non-negotiables for me, and the fourth is a very strong preference. In the future, I'd really like to see:
- support for symbolic links within folders
- built-in dictionary and encyclopedia capabilities (preferably for multiple languages and with the ability to add your own dictionaries and cross-reference a word in multiple dictionaries)
- The ability to correct a misspelled word or edit something in a non-write protected book.

Ironically I could live without the SD card if the reader came with 4 GBs+ of user usable storage. Other than that, #4 and #2 is what really settled my decision on going with the PocketPro instead of the Sony PRS-300.